6.111 Lecture # 18
More Video
MC6847 Display Controller
Obsolete but useful
All Points Addressable
Digital System Side
1
MC6847 with Character ROM
Note the hardware hack
To get around a bug in way /HS is generated
(The first /HS of a frame is AFTER the first row of dots)
2
Black and White (or Green)
Simple way of driving monitor
Has composite sync on the video
Adjust pot (top) so bottom of sync pulse has V=0
3
Color Output
Need to decode the 'chroma' outputs
Comparators on left decode
Gates on right drive RGB
AM25LS32 has fast comparators
'S38 is open collector NANDs
4
Vertical Sync Generation
MC6847 generates blanking but not the vertical sync pulse
Here is one legitimate use for one-shots
RGB Cable
Pin 1
Intensity
Pin 2
Reg
Pin 3
Green
Pin 4
Blue
Pin 5
GND
Pin 6
GND
Pin 7
HSYNC
Pin 8
VSYNC
5
LM1882 Sync Generator
Programmable, flexible part
6
Sync Generator: Setup
Store timing information in registers
Example
256 pixels wide
256 lines
5 MHZ clock (probably not typical)
7
Sync Generator: Register Contents
8
Sync Generator: Physical Setup
LM1882 must be loaded on power up
Use a ROM (PROM) to hold configuration
Your MCU or FSM must do the programming
9
Sync Generator: Timing of Config
This is "Manual Addressing" mode
See data sheet for more
10
Sync Separator
Reverse Direction
Generate Composite Sync from Video
Generate separated sync signals too
11
Getting Information Here to
There
Full Handshake: Timing
No info gets lost
Parallel Interface,
Full Handshake
12
Serial Interface
RS-232 is a serial interface standard
What is shown here is TTL signal
RS-232 levels are inverted from this
13
UART
Universal Asynchronous Receiver/Transmitter
Increasingly less common devices
Example is the AY-3-1015D (now obsolete but useful)
14
AY-3-1015D Transmit Section
15
Transmission Lines
Signals travel on wires
Attenuation -- losses
Reflections -- affected by terminations
16
Signal Propagation
Pulses traveling on the line
Voltage and Current
Ratio of voltage to current is 'characteristic impedance'
Sign of that ratio is direction of propagation
Propagate at < C (speed of light)
17
Pulses are absorbed if receiving end is matched
If not matched, pulse 'reflects'
Sign of refleced wave depends on impedance:
18
Characteristic Impedance Demo
Reflections depend on terminating impedance
Can be minimized by terminating correctly
19
Crosstalk Demo
Flat Ribbon Cable
Similar to kit interconnect cables
Wires situated next to each other
Capacitive and inductive coupling
Crosstalk minimized by alternating wires
Ground - Signal - Ground - Signal ...
20